CVE-2021-34193 – CVE-2021-34193 is a stack overflow vulnerabilit... (How to Fix)

Q: What is the severity of CVE-2021-34193?

CVE-2021-34193 has a CVSS score of 7.5 (HIGH). CVE-2021-34193 is a stack overflow vulnerability in OpenSC smart card middleware that allows remote attackers to execute arbitrary code or cause denial of service via crafted APDU responses. This affects systems using OpenSC for smart card authentication before version 0.23. Organizations using smart cards for authentication or cryptographic operations are primarily affected.

📋 TL;DR

CVE-2021-34193 is a stack overflow vulnerability in OpenSC smart card middleware that allows remote attackers to execute arbitrary code or cause denial of service via crafted APDU responses. This affects systems using OpenSC for smart card authentication before version 0.23. Organizations using smart cards for authentication or cryptographic operations are primarily affected.

💻 Affected Systems

Products:

OpenSC

Versions: All versions before 0.23

Operating Systems: Linux, Windows, macOS, BSD systems with OpenSC

Default Config Vulnerable: ⚠️ Yes

Notes: Any system using OpenSC for smart card operations including PKI authentication, digital signatures, or encryption is vulnerable.

📦 What is this software?

Opensc by Opensc Project

View all CVEs affecting Opensc →

⚠️ Risk & Real-World Impact

🔴

Worst Case

Remote code execution leading to complete system compromise, privilege escalation, or persistent backdoor installation on affected systems.

🟠

Likely Case

Denial of service causing smart card authentication failures, system crashes, or service disruption for smart card-dependent applications.

🟢

If Mitigated

Limited impact with proper network segmentation and smart card response validation, potentially causing only application crashes without system compromise.

🌐 Internet-Facing: MEDIUM - Exploitation requires smart card interaction which typically occurs on internal networks, but exposed smart card services could be vulnerable.

🏢 Internal Only: HIGH - Smart card middleware is commonly deployed internally for authentication, making internal systems primary targets.

🎯 Exploit Status

Public PoC: ⚠️ Yes

Weaponized: LIKELY

Unauthenticated Exploit: ⚠️ Yes

Complexity: MEDIUM

Multiple fuzzing reports demonstrate exploitability. Attack requires sending crafted APDU responses to trigger the overflow.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 0.23 and later

Vendor Advisory: https://github.com/OpenSC/OpenSC/releases/tag/0.23

Restart Required: Yes

Instructions:

1. Download OpenSC 0.23 or later from GitHub releases. 2. Stop all smart card services. 3. Install the new version following platform-specific instructions. 4. Restart smart card services and verify functionality.

🔧 Temporary Workarounds

Network segmentation

all

Isolate smart card services from untrusted networks and implement strict firewall rules.

Disable vulnerable features

all

Temporarily disable smart card authentication if not critical, using alternative authentication methods.

🧯 If You Can't Patch

Implement strict input validation for APDU responses at application layer
Deploy intrusion detection systems monitoring for abnormal smart card traffic patterns

🔍 How to Verify

Check if Vulnerable:

Check OpenSC version with 'opensc-tool -v' or 'pkcs11-tool --module opensc-pkcs11.so -I' and verify version is below 0.23.

Check Version:

opensc-tool -v

Verify Fix Applied:

After patching, verify version is 0.23 or higher using same commands and test smart card functionality.

📡 Detection & Monitoring

Log Indicators:

Application crashes in OpenSC-related processes
Unusual smart card authentication failures
Stack overflow errors in system logs

Network Indicators:

Abnormal APDU response patterns
Unexpected smart card protocol traffic from untrusted sources

SIEM Query:

source="*opensc*" AND (event_type="crash" OR error="stack overflow" OR message="APDU")

📊 Metadata

CVE ID: CVE-2021-34193

CVSS v3 Score: 7.5 (HIGH)

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CWE: CWE-787

Published: August 22, 2023

Last Updated: November 3, 2025

🔗 References

https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=27719 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=28185 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=28383 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=28768 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=28843 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=28855 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=29912 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=30112 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=30800 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=31448 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=31540 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=32149 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=27719 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=28185 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=28383 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=28768 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=28843 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=28855 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=29912 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=30112 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=30800 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=31448 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=31540 Issue Tracking,Mailing List
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=32149 Issue Tracking,Mailing List
https://lists.debian.org/debian-lts-announce/2024/12/msg00026.html

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